When a well such as an oil well is drilled, a bore hole is formed. This bore hole may be largely vertical or may have non-vertical sections. This bore hole is typically formed by rotary drilling.
FIG. 1 schematically illustrates a cross section of a section of a typical production well, for example for oil or gas production, after completion. After the bore hole is drilled into the ground 10, one or more casings 12, 16 are inserted into the bore hole. The casing(s) may be provide to protect aquifers, to provide pressure integrity, to ensure isolation of production formations and to prevent the bore hole from collapsing on itself. Typically a casing is inserted in discrete lengths of pipe that are coupled together, either by screwing adjacent sections together or by other suitable means. Where there are multiple casings a smaller diameter casing may be installed in a section of the wellbore where there is also a larger diameter casing. The smaller diameter casing may run through the whole length of the wellbore having the larger diameter casing or just part of such section, for instance the start of the smaller diameter casing being supported from the larger diameter section.
After a casing is located in the borehole it is typically cemented in place, at least for part of the depth of the well to seal the casing into the ground. Typically concrete 14, 16 is forced down the relevant casing, and out of the end, such that the concrete back fills the annular space between on the outer surface of the casing and its surroundings. Thus in the example shown in FIG. 1 larger diameter casing 16 is located in the well bore. Concrete would then be forced down the large diameter casing to back fill the space between casing 16 and the ground 10. Once this is cemented in place there may be further drilling and later the smaller casing diameter casing 12 may be inserted. This casing may also be sealed in place by cementing, but in this case the cement will back fill the void between the two casings 12 and 16.
There may be various types of casing used in a well. For example casing 12 could be a production casing. A production casing may be one of the last casings to be installed. In use therefore the interior of the production casing represents a pathway between the bottom of the well and the well head. Product could be carried to the surface via the production casing but usually production tubing is installed within the production casing and the production tubing carries the product. The additional, larger diameter, casing 16 may be an intermediate casing, a surface casing or a conductor casing. An intermediate casing may be used to protect against caving of weak or high pressured formations and to enable the use of drilling fluids of different density necessary for the control of lower formations. A surface casing may be used to protect fresh-water aquifers onshore, and to anchor blow out preventers (BOPs). A conductor casing may be used to support surface formations, and may be set soon after drilling has commenced since shallow parts of the bore hole can quickly cave in. As discussed the additional casing is inserted into the bore hole prior to the insertion of the production casing 12 and is set in place with concrete 18.
Although only a single casing 16 is shown in addition to the production casing 12, it should be apparent that multiple additional casings may be provided, with each additional casing being sealed in place with concrete.
When the concrete 14 around the production casing has set and the structure of the well is complete, a flow path is created from the production casing to a reservoir in the ground containing the fluid product. The most common method uses a jet perforating gun equipped with shaped explosive perforation charges. Fracturing may then be performed in order to increase the fluid flow from the reservoir to the production casing, thereby increasing the productivity of the well.
As the casings are generally inserted in discrete lengths that are coupled together, these joins can cause areas of concern in well formation and operation. Given that the casings are cemented into place on completion of the well and can only be removed by further drilling, it is desirable to check the integrity of these casings before they are cemented into place. Also, given the high cost of a well installation and the potential environmental and economic consequences involved if a well were to leak, it is desirable, to be able to monitor the integrity of a well. In particular, it is desirable to monitor for leaks in a well.
Embodiments of the present invention aim to address at least some of the above problems to some extent.